One of the most senior members of the controversial Islamist Hizb ut-Tahrir movement, the group which Tony Blair pledged to ban after the 7 July bombings, has dramatically defected.

Maajid Nawaz, a Briton who was jailed in Egypt for four years in 2002 and allegedly tortured after being accused of reviving a banned political group and plotting to overthrow the government, says that he no longer agrees with the movement's attempts to transform Islam into a narrow political ideology.

The departure of Nawaz, whose reasons for quitting have been posted on Hizb ut-Tahrir's British website, has already inspired a furious debate within the group. Although he left two months ago, he has only now made his decision public, and will be interviewed later this week on BBC2's Newsnight programme.

According to Nawaz, who joined the movement when he was 16, he reconsidered his membership during his imprisonment. Insisting he had not abandoned his opposition to the conduct of Arab leaderships or his opposition to the Iraq war, he added that he could no longer agree with its thinking.

Nawaz's defection is a major blow to the movement. Despite being only 29, he was recognised globally for his work in founding new groups abroad.

Equally important is likely to be his reason for leaving Hizb ut-Tahrir, a closely argued rejection based on a revisiting of the key legal argument that has been deployed by Islamists - including al-Qaeda - that Arab governments operating under non-Islamic (kufr) law should be removed.

He added, however: 'I do not wish my critical voice to be exploited to support the call for proscribing Hizb ut-Tahrir. Rather, those that are well versed in both party ideology and traditional Islamic sciences are sufficiently equipped and duty-bound to redress the phenomenon of politically inspired theological interpretations.

'I would also state that my political stance against the tragic invasion of Iraq remains unchanged. Furthermore, these words should not be taken to mean that I support the brutal policies of the Muslim world's dictators. Rather, political engagement is the civic duty of all who are able. Finally, I impress upon all people that Islam today is not in need of a politically inspired modernist reformation, which is actually the cause of our current crisis, rather a counter-reformation and a return to its true essence by Muslims insisting that their religion is not used merely to serve narrow political agendas.

'I spent much of my time as a political prisoner evaluating and studying Islam. It became clear to me that rather than being the sole vanguard that represents an Islam that even Muslim jurists have misunderstood, Hizb ut-Tahrir is inspired by a political ideology.'

While some of the responses to his move yesterday were abusive, some welcomed the debate. 'He has an argument,' commented one senior member. 'He appears to back [it] with texts/evidences. So let a discussion develop, a discussion free from useless feeble statements.'